The present invention relates to interconnect devices used in electronic assemblies, and more particularly, to pin grid arrays used to connect a large number of contacts from one circuit to another circuit.
Microelectronic circuits such as microprocessors are frequently packaged so that their large number of electrical contacts can be accessed via a pin grid array (PGA). The pin grid array comprises rows and columns of tiny conductive pins that extend generally perpendicular from the planar underside of a thin rectangular housing. It is not uncommon to have well over two hundred pins in a pin grid array. The housing of the microprocessor is made of ceramic or plastic and encases an integrated circuit chip. Microscopic wire leads electrically connect the upper ends of the pins to individual contacts on the chip. The pins may be plugged into corresponding receptacles in a socket connector mounted on a circuit board. The pin grid array has the advantage of providing a reliable mechanical and electrical interconnection between the microprocessor and the circuit board, while at the same time allowing the microprocessor to be removed for repair or replacement. For example, a mother circuit board for a personal computer may have a microprocessor with a pin grid array that can be unplugged so that an enhanced microprocessor, e.g. one having a math coprocessor, can be installed in its place.
An alternative to the pin grid array is the ball grid array. It comprises rows and columns of tiny solder balls attached to the underside of a microprocessor. These solder balls register with corresponding conductive pads or traces on the circuit board. Mechanical and electrical connection is achieved by subjecting the board and microprocessor to infrared or convective heating to achieve solder reflow. While this is a reliable method for surface mounting microelectronic components on a circuit board, the components cannot be easily removed for repair or replacement. However, it is a less expensive approach than the pin grid array since the cost of pins and socket connectors is eliminated.
Pin grid arrays have also been used on separate carrier boards to interconnect a large number of electrical contacts on a first circuit board to a second circuit board. FIG. 1 is an exploded diagrammatic view illustrating a prior art technique widely used in the personal computer industry to connect a first plurality of electrical contacts on the underside of a multi-chip module (MCM) board 10 to a second plurality of corresponding electrical contacts on computer mother board 12. A carrier board 14 is provided that has a plurality of pins 16 that extend through the carrier board. The pins 16 are arranged in a grid array and their upper ends are soldered to corresponding pads or traces on the underside of the MCM board 10. The lower ends of the pins 16 can then be plugged into a socket connector 17 attached to the upper side of the mother board 12. The socket connector 17 provides a means that can be mounted on, or directly to, the mother board 12 for individually receiving and providing electrical connection with the pins 16. The prior art pin grid array interconnect arrangement of FIG. 1 allows computer manufacturers to make a common mother board that can be utilized to make a variety of different computer configurations. It also allows computer users to easily upgrade their systems, e.g. to run more efficiently and at higher speeds, by simply plugging a suitable MCM 10 into the mother board 12.
The prior art circuit board interconnection technique illustrated in FIG. 1, while serviceable, has reliability problems. FIG. 2 illustrates a greatly enlarged vertical cross-sectional view of a so-called "butt joint" solder connection 18 between the upper end of one of the pins 16 and a conductive pad 20 on the underside of the MCM board 10. A pattern of solder is typically screened onto the conductive pads 20 on the MCM board.
When this solder undergoes reflow, a fillet 22 of solder is formed around the upper end of the pin 16. However, the fillet 22 is only slightly larger than the radial width of the pin 16. In addition, the solder joint 24 between the upper end of the pin 16 and the conductive pad 20 is relatively narrow in vertical height. The result is a delicate mechanical interconnection between the pin 16 and the conductive pad 20. It is often necessary to straighten one or more pins 16 in the grid array on the carrier board 12. This can lead to fractures through the corresponding fillet 22 and joint 24, resulting in intermittent electrical contacts or open circuits that impair proper operation of the associated circuits. The MCM board 10 and mother board 12 are typically made of FR-4 material, a laminate of fiberglass, epoxy and etched copper circuit traces. The carrier board 14 is typically made of a high temperature thermoplastic. The differences in the coefficients of thermal expansion of the different materials can stress the solder connections between the upper ends of the pins 16 and the conductive pads 20 on the underside of the MCM board 10, leading to fractures through the fillet 22 and joint 24, resulting in intermittent electrical contacts or open circuits. Lateral loads on the MCM board 10, such as pushing by the user, can also break one or more of the solder connections between the pins 16 and the MCM board 10.
Reliability problems with the prior art circuit board interconnection technique illustrated in FIG. 1 are compounded if there is any substantial departure from true coplanarity between the MCM board 10 and the grid array of pins 16. FIG. 3 illustrates a greatly enlarged vertical cross-sectional view of an alternate defective solder connection 26 between the upper end of one of the pins 16' and the conductive pad 20' on the underside of the MCM board 10. The solder connection 26 occurs when lack of true co-planarity between the MCM board 10 and the carrier board 12 places the upper end of the pin 16' too far away from its corresponding conductive pad 20'. An excessively elongated fillet 22' of solder is formed around the upper end of the pin 16'.
In addition, the solder joint 24' between the upper end of the pin 16' and the conductive pad 20 has been excessively elongated. The result is an even more delicate mechanical interconnection between the pin 16' and the conductive pad 20.
It would therefore be desirable to provide an improved pin grid array interconnect system and method that would overcome the above-noted deficiencies of the prior art technique illustrated in FIGS. 1-3.